This contract was awarded for the purpose of evaluating the efficacy of a live, attenuated varicella zoster virus (VZV) vaccine in high-risk children and adults. Varicella (chicken pox) is a highly communicable infectious disease which is common during childhood. For most children it is a self-limited infection without serious sequelae. However, in children with acute lymphocytic leukemia (ALL) or other malignancies, primary chicken pox can be a extremely serious and potentially life-threatening illness. In this study, the contractor has systematically evaluated a new and promising vaccine to prevent chicken pox. The purpose of this renewal is to continue the studies of this vaccine in the leukemic population and to obtain data in other high-risk groups, particularly other children with solid tumors and healthy adults who are seronegative. The contractor will establish the safety and efficacy of this vaccine in the leukemic population. Of particular importance is to determine if the vaccine has the capacity to be transmitted to other children. In addition the contractor will focus special attention of the capacity of the vaccine virus to establish a latent state and to determine the frequency of reactivation and its consequences.